In nuclear water reactors, such as pressurized water reactors and boiling water reactors, it is typical for oxide scale-containing radionuclides to be adhered to or generated on surfaces of structures, e.g., components and related parts or piping, which are in contact with fluid, e.g., coolant, over extended time periods during reactor operation. Nuclear water reactors are generally thermal reactors in which water is used as the coolant. The water passes through numerous metal components, such as stainless steel and Alloy 600, Alloy 690 or Alloy 800 conduits. Even though these materials of construction are highly resistant to corrosion, thin oxide coatings (or films) develop over time on the surface areas of components and related parts or piping which are wetted by the coolant during power operation of the reactors. It has been found that portions of the oxide coatings may dissolve into the coolant and may be transported by the coolant throughout the systems, e.g., reactor coolant system. The accumulation of scale and deposits on the surfaces of the structures can have an adverse impact on the operational performance and integrity of the structures.
The primary side, e.g., reactor coolant system, surfaces of components in pressurized water reactors (PWRs) and the internal components in boiling water reactors (BWRs) contain radionuclides which are formed during reactor operation. The radionuclides are typically radiocobalt in a nickel ferrite lattice. A variety of systems and methods have been developed in the art to remove or reduce the presence of radionuclides on internal components of BWRs and primary side surfaces of PWR components. It is known in the art to reduce radionuclides by chemical injection. For example, a zinc compound can be injected into the coolant water of a nuclear water reactor at full power to reduce or remove radionuclides. Further, it is known to employ a high temperature process wherein a cleaning solution is prepared, heated and injected into the entire system or injected locally. Many of these known decontamination methods have proven to be cumbersome and require handling of high temperature fluid and multiple chemical steps, such as oxidation and reduction.
Thus, known radioactive decontamination typically involves elevated temperature dissolution or mechanically induced turbulence or a combination thereof depending on the intended component to be decontaminated. Further, known techniques require the flow of high temperature fluids, mechanical hand cleaning the case of coolant pumps) and a length of time under mechanical agitation. Furthermore, these techniques require chemistry conditions to be aggressive, e.g., switching from oxidizing to reducing conditions. Generally, known techniques employ temperature, pH and redox potential shifts for the removal or reduction of radionuclides and these techniques are rarely performed at a nuclear reactor plant due to the amount of radioactive waste generated.
It is desired in the art to develop a method for localized decontamination and deposit removal which does not require added heat, e.g., can be conducted at ambient temperature, or liquid flow.